Automatic transmissions may employ single or multiple planetary gear sets. Each planetary gear set has a central or sun gear; a ring or internal gear; and a carrier which supports a plurality of pinion or planet gears that are meshingly interposed between the sun and ring gears. Selective connection of one member of a planetary gear set with another member to effect simultaneous rotation of those members and/or selective connection of one or more members of a planetary gear set with nonrotatable structure, such as the transmission housing or case, to preclude rotation of that member, determines the drive ratio provided by the transmission. Torque transfer devices which effect a connection between two rotating members of a planetary gear set are generically designated as clutches, and torque transfer devices which effect a connection between a rotating member of a planetary gear set and a nonrotatable member such as the transmission case are generically designated as brakes.
Torque transfer brakes have historically employed brake bands selectively to retard, stop and/or preclude the rotation of one or more of the rotatable planetary gear members in a vehicular transmission. A transmission brake band typically circumscribes the cylindrical reaction surface on the periphery of a drum member presented from at least one of the rotatable components in a planetary gear set. One end of the band is fixedly anchored, as to the transmission case, and the other end of the brake band presents an input assembly which interacts with a linearly displaceable servo-apply pin that actuates the brake band by displacing the input assembly in a manner that either constricts or expands the circumferential diameter of the brake band in order, respectively, for the brake band frictionally to grip or release the cylindrical reaction surface of the drum member.
The servo mechanism employed to operate each brake band generally incorporates a hydraulically operated piston assembly. Historically, the manufacture of the transmission case within which the planetary gear set was to be housed was further complicated because the transmission case required a boss, the interior of which was accurately machined to receive the servo mechanism. For example, the boss was machined to provide the necessary piston chamber within which an actuating piston assembly could reciprocate to effect axial translation of a servo-apply pin, and thereby operate the brake band assembly.
Transmission cases have rather universally been, and continue to be, metallic castings. Castings do, on occasion, incorporate voids, but even microscopic voids, which are generally considered as merely contributing to the porosity of the casting, can require additional labor to prevent leakage paths through the casing.
As should be readily apparent, the structural properties of strength and hardness required to make an acceptable transmission case are not necessarily conducive to providing a readily machinable casting. In fact, variations in the sectional thickness of a casting, and particularly a casting having the complexity of a transmission case, can cause localized hard or soft spots. Although one might ideally desire a more homogenous casting, such localized variations in the physical properties of the casting may not themselves negate the suitability of the casting to serve as a transmission case. Unfortunately, however, such localized variations can adversely affect the ability of the casting to be acceptably machined.
For example, the reaction of a machine tool against a localized hard spot can cause microscopic, if not macroscopic, grooves which could well preclude the effective sealing of the pressure chamber required for the piston assembly in a servo mechanism built into the transmission case, and such flaws might not be identified until after at least partial assembly of the transmission. But even if any such flaws were detected at the earliest possible opportunity, the result could well be a rejected transmission case.
Hydraulically operated servo mechanisms for vehicular transmissions can be complex and relatively expensive to manufacture--particularly according to prior art methods whereby the servo mechanisms had to be linearly assembled within a cavity machined into a boss on the transmission case. Such historic assembly techniques are labor intensive and unduly contribute to the cost of the transmission.